The invention relates to envelope detectors and, more particularly, to a high performance envelope detector which provides controlled decay envelope detection.
Typically, classic envelope detectors employ diodes and RC networks to obtain a DC voltage level that represents a congregation of the momentary peak amplitudes of an amplitude modulated (AM) signal. The response of such detectors to a negative-going voltage pulse conforms to the response of an RC network, i.e., is an exponential decay voltage curve. The degree or rate of decay depends upon the RC time constant and thus may be selected to provide a faster or slower decay, as desired.
Envelope detector circuits are variously used in many electrical systems, to detect the DC voltage level that represents the peak amplitudes of a corresponding signal. For example, in professional audio tape recorders, envelope detector circuits are used to obtain a constantly varying DC level corresponding to the amplitude of the envelope of an amplitude modulated reproduced signal. The DC level is used in the signal processing automation circuits of the recorder such as in automatic biasing and equalization checking circuits.
As is commonly known, tape dropouts occur when recording signals on magnetic tape medium due to scratches and dirt on the tape, head-to-tape conditions, etc., which dropouts in effect define a loss of signal, i.e., a negative change in the DC level of the envelope. It is not desirable to detect the DC level change due to dropouts, since they do not represent the true DC signal level and will cause an error in the measured signal and thus in the subsequent processed signal.
In the prior art envelope detectors of previous mention, the response of the detector is slowed by selecting the proper RC time constant, whereby the dropouts are not detected. However, then the detector also fails to respond to the faster envelope DC level changes.
Conversely, if the detector is fast enough to respond to, and thus correctly detect, the faster envelope DC level changes, then it will also detect the unwanted changes caused by the dropouts. In addition, the faster envelope detector also generates a DC signal with a more pronounced stepped effect corresponding to each momentary peak of the AC carrier signal.